1. Field of the invention
The invention relates generally to motion picture sound track recording and play-back, and more particularly to such systems providing improved signal-to-noise ratio and security against film piracy.
2. Description of the Prior Art
In the prior art, it has been standard practice to provide a sound track between the picture frames and the edge of the film. This sound track is light modulated in either a variable density, constant width, or a constant density, variable width format. Play-back is accomplished by continuously converting the instantaneous light intensity from a flood source through a slit or light gate. That intensity, of course, varies with film density or track width as the film passes the light gate. A photocell converts the variable light intensity to electrical form for appropriate filtering and amplification before driving a speaker system providing the last step in sound reproduction.
The prior art systems have been found to be particularly vulnerable to noise due to dust, scratches, wear of film surface and other amplitude influencing factors. When the sound track is subdivided into two tracks for stereo recording and reproduction, the inherent signal-to-noise ratio declines further. The encountered noise manifests itself as clicking, popping and hissing sounds.
A logical outgrowth of the industry's efforts to improve cinema theater sound quality was the use of magnetic tape sound track (or tracks) processed onto the film in the sound track area. Although the quality of sound thereby obtained is very good, the production and processing costs are very high. First the picture must be shot and the sound, synchronously, but separately recorded. The film is subsequently processed to develop the picture, then to affix the magnetic track (tracks) and finally to record the sound. The sound recording is thus very expensive and theaters showing such films require additional equipment not compatible with the traditional optically recorded sound tracks (academy sound). Still further, the magnetically recorded sound tracks are subject to damage due to extraneous magnetic fields encountered (in shipment, for example) and therefore must be specially handled.
In a more economically successful approach, appreciable signal-to-noise improvement has been obtained in the rather widely accepted, so-called Dolby system. Basically the Dolby system envolves pre-emphasis on recording and de-emphasis on reproduction. This pre-emphasis is undertaken in frequency regions in which noise is a particular problem, subsequent de-emphasis operating to reduce the desired frequencies to normal, the proportional reduction of noise frequencies thus tending to suppress noise in the overall. Such systems are generically referred to as companders, however the Dolby system has produced improvement over the basic compander concept.
The need currently exists for quantum step improvement in the motion picture sound recording and play-back art. Improvements are far more dramatic and economically significant when they preserve the compatibility of apparatus to show films recorded in the very widely used "Academy" format.
The manner in which the invention provides for very significant signal-to-noise improvement with low cost apparatus and with the highly desired film compatibility will be evident as this description proceeds.